This invention relates generally to diagnostic and control channels for electrical components. More particularly, this invention relates to multiplexing an ignition coil circuit node to perform a diagnostic function and a control function.
Gasoline internal combustion engines now commonly use a single ignition coil for each cylinder. The ignition coil is frequently configured for mounting directly atop a spark plug screwed into the cylinder head. Such an ignition coil arrangement is commonly known as a coil-on-plug arrangement.
A power transistor within an engine control module (ECM) generally conducts current flow through the primary winding of an ignition coil during a dwell period, after which the spark plug fires. The ECM also generally contains a microprocessor that executes software to diagnose the performance of the ignition coil. This diagnosis is commonly performed by measuring the voltage across the power transistor, which is representative of the voltage across the primary winding of the ignition coil and indicative of ignition system performance.
The ECM power transistor develops heat, however, making it desirable to locate the power transistor outside of the ECM and away from the microprocessor. A common location for the power transistor is on the ignition coil where it is in close proximity to the primary winding. Such an arrangement presents at least two new problems, however. The first problem lies in the course of events should the control wire to the power transistor become shorted to a high or low voltage source, such as battery or ground, respectively. Without additional circuitry, either the ECM, the power transistor, or both, could become damaged and unserviceable by excessive current flow and power dissipation.
The second problem lies with reliably diagnosing performance of the ignition coil. A solution to the diagnostic problem has heretofore required diagnostic wiring, additional to the control line for sending a dwell pulse to the power transistor, to be connected between the ECM and the ignition coil/power transistor assembly (hereinafter referred to as an ignition coil assembly). The additional wiring carries an electrical signal from ignition coil assembly back to the ECM so that it may perform diagnostics on the assembly and its performance. This additional diagnostic wiring creates added expense through higher connector pin counts and added conductors. The additional wiring also increases the risk of system failure by failed connections.
It is therefore one aspect of the invention to provide an ignition coil assembly having an integrated driver where serviceability of the assembly is tolerant of a driver control line being short circuited.
It is yet another aspect of the invention to provide an ignition coil assembly having a common signal line for both a dwell pulse and diagnostic information.
In accordance with the aforementioned aspects, the present invention provides an ignition coil assembly for providing ignition energy to a spark plug in accordance with a dwell pulse and transmitting diagnostic data related to an ignition event occurring after the dwell pulse. The assembly has an ignition coil with a spark plug terminal adapted to mate with the spark plug, and a transistor for conducting current flow through the primary winding of the ignition coil. The current flow is in accordance with the dwell pulse, which arrives over a signal line. A diagnostic block receives at least one electrical signal from the ignition coil and derives diagnostic data therefrom for transmitting over the same signal line in the absence of the dwell pulse.